Ceramic materials are commonly provided with a natural porosity determined by the composition and the grain structure of the raw materials, and by the driving out of the water during drying and firing. This natural porosity can be insufficient for application situations where it is important to assure a good thermal and electrical insulation, in addition to refractory properties, and where a reduced weight is required.
Various ceramic products with higher porosity levels have been developed in the art; the incorporation of pores is commonly obtained by means of so-called porogens, i.e., pore forming materials such as graphite, polymer beads or fibres, which burn, evaporate or gasify upon heating without leaving residues. Porogens are commonly premixed with the ceramic precursors and volatilize during the firing step, thus leaving pores in the ceramic body which correspond to the space originally occupied by the porogen particles.
The obtained porous ceramics, having reduced density and weight, are commonly used in a wide range of applications, such as tableware, sanitaryware, tiles, insulators, and various building and construction materials.
For instance, U.S. Pat. No. 5,171,721 describes porous ceramic sheets, which are obtained by mixing a ceramic powder with planar or elongate flakes, such as graphite, starch and sugar, which disappear upon application of heat to provide generally planar or elongate pores corresponding to the flakes. The sheets are used as a protective layer for electrodes. U.S. Pat. No. 6,057,030 discloses porous ceramic bodies characterized by a plurality of interconnected pores, obtained by mixing a powdered ceramic component, such as alumina, with pore-forming polymeric beads, such as polymethylmethacrylate beads, compressing the obtained mixture and finally heating it to vaporize the polymeric beads.
The mechanical strength of the fired ceramic is usually directly related to the ceramic's density, i.e. dense ceramics are generally stronger than porous ceramics. In fact, while the introduction of voids in the ceramic material has very positive effects on thermal and electrical insulation properties of the material and also allows a reduced weight, the presence of voids may also lead to a disadvantageous strength reduction. Dramatic reduction of modulus of rupture can be experienced when high levels of porosity are introduced in the ceramic, and in particular open porosity. Open porosity not only reduces melt strength, but also allows water and moisture to penetrate in the ceramic body, which is undesired for many traditional ceramic applications; in fact, open porosity is determined by the pores which are connected to the surface of the ceramic and linked with each other.
As known in the art, the decrease in mechanical strength may be partially avoided by controlling homogeneity of the pore distribution. US Statutory Invention Registration H48 of Apr. 1, 1986 describes a porous ceramic article obtained by using thermally degradable beads, such as polystyrene foam beads, in the presence of a binder capable of taking a set, such as an epoxy resin; after forming the desired ceramic article, heat is applied to degrade and volatilize both the beads and the binder.
US2006/0228247 describes the use of homogenizing agents, such as alcohols, isoparaffinic solvents or polyethyleneglycol, which maintain a uniform distribution of the pore-forming agents within a metal or a ceramic mass. The pore-forming agents are metal salts which are removed by water extraction after the desired article has been formed, thus avoiding high temperature vaporization or other severe measures that may alter the character of the final porous article. Nevertheless, the obtained porous ceramics are still unsatisfactory as regards mechanical strength.
Further problems associated to the use of high amounts of porogens are caused by the quick decomposition and combustion of the porogens upon heating, which may lead to cracking in a moulded body. An attempt to solve this problem is disclosed in US2005/0161849, wherein the pore-forming material is composed of inorganic particles contained in hollow organic polymer particles, such as polymer micro-balloons; the inorganic particles are transferred into the polymer particles by degassing. The presence of inorganic particles in the pore-forming material decreases the relative amount of organic material, thus avoiding the occurrence of local heat shocks during the heating step.
Therefore, there is a need for developing ceramic porous materials having decreased weight and excellent thermal and electrical insulation properties, while at the same time maintaining good mechanical strength.